Methods are known from the prior art, in which the operator uses an electromechanical controller to control the operating device of the load-handling element of a fork-lift truck. On the basis of the analog control voltage formed by the electromechanical controller, an electrically controlled directional control valve operates the operating device. A solution according to the prior art is presented, for example, in the book Vehicle and Implement Hydraulics (Ajoneuvo-ja työkonehydrauliikat (in Finnish)), particularly on pages 74-77, (Louhos, P. & Louhos J-P., 1992. Ajoneuvo-ja työkonehydrauliikat. Kangaslampi: Karjala-dealers KY. 268 pp.) The operating device operates the load-handling elements, which can be, for example, the forks or grabs of a truck. The operating device and directional control valve are part of a control element. When using such an apparatus to handle loads which, for example, should be lifted by gripping them from the sides with grabs, the loads can be damaged by excessive pressure. When the loads are of different sizes, during lifting they should be gripped with a precisely suitable force, which varies from load to load. When using such an apparatus to handle loads, the operator is very important, as they adjust the compressive force by using an electromechanical controller to control the operating device. If the operator keeps the electromechanical controller in the ‘on’ position for too long, the grab will compress the load with an excessive force. The apparatus described above is used in Linde E 14—20-type fork-lift trucks, among others. The apparatus described is also used in many other fork-lift trucks, in which there is electrical pre-control. In such a known device, the operating device is controlled using an analog control voltage formed by an electromechanical controller, by means of an electrically controlled proportional valve. The proportional valve permits, for example, exactly the desired gripping pressure or lifting speed.
In FIG. 3, the solid lines are used to show how the analog control voltage depends on the position of the controller. The analog control voltage of the potentiometer can be, for example, 5, 12, 14, or 24-V direct current. In an electromechanical controller, for example in a joystick, there can be one or more sliders, i.e. potentiometers. When the controller contains several potentiometers, they can be in different directions, so that when the voltage of one increases the voltage of another decreases. This is precisely the case in the graph shown in FIG. 3. When the first voltage, which is shown by the line 10, rises, the second voltage, which is shown by the line 10′, drops. In the case of several potentiometers, the voltages can also be stepped, in which case one will be slightly more than another. If the difference in the voltage coming from the sliders is unexpected, the operating system terminates the control for safety reasons.
The signal coming from the electromechanical controller can be cut and replaced with an entirely new signal. The new signal comes from a new controller. The electromechanical controller is then replaced with a more intelligent control system. Cutting the signal coming from the electromechanical controller and replacing it with a new signal is, however, in no way without its problems, as a difference can easily appear in the voltages coming from the sliders. The operating device interprets the difference in question as an error and terminates the control for safety reasons. For this reason, cutting a signal coming from the electromechanical controller and replacing it with a new signal is very challenging. Even though the signal monitored by the operating device may only deviate for a short moment, the monitoring may be timed for exactly that moment. In some systems, resetting the error is difficult and must be performed by a maintenance technician.
On the other hand, apparatuses are known from the prior art, in which digital control signals are edited. In addition, apparatuses are known from the prior art, in which the control pressure is adapted. Examples of such apparatuses are publications JP 7-109095 and JP 5-238686. The apparatuses in question permit the operating device to be controlled in such a way that the load is not pressed too tightly, for example. Such apparatuses, which alter the control pressure of the digital control signal, are easy to install during the manufacture of the truck. The entire control system is then manufactured taking the totality into account. However, there is a problem with trucks that have already been manufactured, in which there is already a control system without the adaptation of a control variable, for example, the control signal or control pressure.